Japanese laid-open Patent Publication No. 2002-156010 discloses a hand-held power tool in which a planetary gear mechanism is utilized as a safety clutch. In a power tool such as a hammer drill, reaction torque acts on the tool body in an opposite direction from the direction of rotation of the hammer bit during hammer drill operation. When the hammer bit is unintentionally locked during hammer drill operation, reaction torque acting on the tool body increases and thus the tool body may be swung. In the known power tool, an outer ring member in the planetary gear mechanism is pressed and held by a contact element including a control means in the form of a brake shoe. When a tool bit is unintentionally locked during drilling operation, the outer ring member held by the contact element is released, so that the tool body is no longer acted upon by reaction torque and avoided from being swung.
In the known power tool, a torque limiter is formed by utilizing the planetary gear mechanism, but the power tool is increased in size due to its structure utilizing the planetary gear mechanism. In this point, further improvement is required.